1. Field of the Invention
The present invention relates to a method of purifying exhaust gas that is discharged from internal combustion engines, which are operated at a fuel-lean air/fuel ratio, such as gasoline lean burn engines, 2-stroke engines, and diesel engines, and contains oxygen in excess to reducing components, such as hydrocarbons, and particularly to a method useful for removing nitrogen oxides.
2. Description of the Prior Art
Internal combustion engines that are operated at a fuel-lean air/fuel ratio, such as gasoline lean burn engines, and diesel engines, (hereinafter referred to as lean burn engines) are advantageous because the amount of fossil fuel required to do a certain work is less in comparison with conventional gasoline engines and the amount of exhaust of carbon dioxide (CO.sub.2), a substance that causes global warming, can be decreased. However, because the exhaust gas contains oxygen in excess to reducing components, such as hydrocarbons (HC), the prior art technique of removing nitrogen oxides (NO.sub.x) is unsatisfactory and therefore its wide propagation is restricted.
Conventionally, with respect to NO.sub.x, nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2) that are considered to cause mainly acid rain and photochemical smog have been remarked in view of the pollution control while nitrous oxide (N.sub.2 O) is considered harmless. However, quite recently, it is pointed out that N.sub.2 O likely causes global warming and destruction of the ozonosphere and it is expected that the exhaustion of N.sub.2 O is required to be restricted in the future.
In recent years, as a method of treating exhaust gas from lean burn engines for transportation means, such as automobiles, it is studied that a catalyst is used to selectively reduce NO.sub.x with HC remaining in the exhaust gas without allowing that HC to react with oxygen present in excess, and various methods are proposed wherein different catalysts are used. For example, there are proposed (1) a method wherein a zeolite catalyst a metallosilicate catalyst, or an aluminophosphate catalyst that have been ion-exchanged with a transition metal, such as Cu and Co, is used (U.S. Pat. No. 4,297,328, and Japanese Pre-examination Patent Publication (KOKAI) Nos. 63-100919, 3-127628, 3-229620, and 1-112488) and (2) a method wherein a catalyst comprising a noble metal, such as Pd, Pt, and Rh, supported on a porous metal oxide carrier, such as zeolite, alumina, silica, and titania, is used (Japanese Pre-examination Patent Publication (KOKAI) Nos. 3-221143 and 3-221144). These catalysts are required to have various characteristics, and since in some cases the exhaust gas from a lean burn engine reaches a temperature of 700.degree. C. or more, and in particular in the case of the exhaust gas from an engine for transport vehicles, such as buses, trucks, and passenger cars, the temperature of the exhaust gas sometimes reaches as high as about 800 to 900.degree. C. temporarily during the operation because the load on the engine changes violently, the catalyst for purifying exhaust gas is demanded to have heat resistance at such a high temperature. In addition, a long-term reliability to the extent of 100,000 to 160,000-kilometer mileage is demanded. However, the catalyst in (1) has defects that the effective temperature range where the removal of NO.sub.x occurs is 350.degree. C. or over, which is relatively high, and that the catalyst is thermally deteriorated drastically in a steam-containing exhaust gas at 650.degree. C. or over, and on the other hand although the catalyst in (2) allows conversion of NO.sub.x at a relatively low temperature of 200.degree. C. to 300.degree. C., the catalyst has problems that the effective temperature range is narrow and a considerable amount of N.sub.2 O is produced concomitantly as a partial reduction product of N.sub.x O.
As a method that offsets the defects of the above single catalyst system and realizes a higher efficient removal of NO.sub.x, various methods are proposed which use a plurality of catalyst systems that are combined stepwise. For example, (1) a method wherein a Cu-ion-exchanged zeolite catalyst is placed in the upper stream of the flow of exhaust gas and a Pt/alumina catalyst is placed in the lower stream (Japanese Pre-examination Patent Publication (KOKAI) Nos. 1-139145 and 4-367713), (2) a method wherein a Ni or Ru/alumina catalyst is placed in the upper stream and a Pt/alumina catalyst placed in the lower stream (Japanese Pre-examination Patent Publication (KOKAI) No. 5-76776, and (3) a method wherein a Pt, Rh, or Pd/alumina catalyst is placed in the upper stream and a Cu/zeolite catalyst is placed in the lower stream (Japanese Pre-examination Patent Publication (KOKAI) No. 5-96132 are known. However, since, in the method in (1), the catalyst in the upper stream and in the method in (3), the catalyst in the lower stream are unsatisfactory in heat resistance, respectively, and in the method in (2), the NO.sub.x removal performance is unsatisfactory in a high-temperature range of 300.degree. C. or higher, all of these methods fail to be used in practice.
Further, a two-stage catalyst system is known wherein a catalyst comprising rhodium or iridium is placed in the upper stream and a catalyst comprising platinum or palladium is placed in the lower stream (Japanese Pre-examination Patent Publication (KOKAI) No. 52-65177). However, this catalyst system is intended to purify exhaust gas from an internal combustion engine that is operated at an air/fuel ratio of about 14.6 where the reducing components and the oxidizing components in the exhaust gas are equivalent, so that the catalyst system is not effective to purify the exhaust gas from lean burn engines that are operated in the presence of excess oxygen and the removal of NO.sub.x proceeds scarcely.
Further, a method wherein the exhaust gas from a lean burn engine is purified by using a catalyst comprising iridium and platinum, in coexistence manner, supported on a metallosilicate carrier is known (Japanese Pre-examination Patent Publication (KOKAI) No. 5-245386). However, in the catalyst used in this method, the light-off (catalyst inlet temperature versus conversion) performance curve of NO.sub.x, HC, and CO is similar to that of a catalyst supported only platinum and any substantial effect due to the action of iridium cannot be observed. The performance of the removal of NO.sub.x is still unsatisfactory. It is assumed that, probably, when exhaust gas comes in contact with such a co-supporting catalyst, out of HC's that are to act as reducing agents for NO.sub.x present in the exhaust gas, more reactive olefinic HC's interact first with platinum that is higher in oxidizing ability, and as a result, N.sub.2 O is produced as a reduction product of NO.sub.x in the low-temperature zone and the olefinic HC's are oxidized to CO.sub.2 completely in the higher-temperature zone.